The present invention is related to the modeling of the interactions between particles and surfaces.
Modeling of the transport of particles around spacecraft in orbit is a poorly developed science. No current models account for the way in which real particles bounce off of surfaces when they impact.
The process by which particle contamination moves throughout a system may be divided into three phases. The first phase is generation and release from a surface, the second phase is transport through air or vacuum, and the third phase is reflection or deposition on a surface. The first two phases have been the focus of prior art, such as the studies referenced in E. N. Borson, “A model for particle redistribution during spacecraft launch,” Proceedings of SPIE Vol. 4774, (2002), pp 67-78, the entire disclosure of which is incorporated by reference herein.
With respect to the generation and release of particles from a surface, many particles are generated by wear between moving surfaces while other particles are “pre-existing” on a surface due to contamination built up over the life of the surface.
Most particle generation on spacecraft occurs during ground processing. Particles shed from personnel, such as garment fibers and skin cells, have a low mass to drag ratio and are transported throughout the clean room by air flow. Abrasion of materials, such as drilling, grinding, and cutting, produces particles with a high mass to drag ratio, and these particles settle close to the point of generation.
Deployable systems such as antenna booms, solar arrays, and covers create materials after spacecraft are in orbit. Actuators create particles at various velocities: pyrotechnical devices release high speed particles, spring cutters release medium speed particles, and sealed wax actuators release slow particles.
Particles may be released from surfaces by vibration or other external forces.
With respect to the transport of particles, drag is the viscous friction effect exerted on a moving particle by the medium it is in. The result is that large particles tend to stray from the flow direction because of momentum but small particles follow the flow direction and are less likely to impact surfaces. The equations of motion of particles suspended in air may be found in A. Busnaina and X. Zhu, “Submicron Particle Motion and Deposition in a CVD Chamber,” Institute of Environmental Sciences Proceedings Volume 2 (1992), pp 1-9, the entire disclosure of which is incorporated by reference herein. As air density drops, the drag also drops, until in vacuum the particles are affected only by gravity. More information regarding particle motion in a vacuum can be found in A. L. Lee, “Particle Dispersion Around a Spacecraft”, American Institute of Aeronautics and Astronautics, Aerospace Science Meeting, 21st, Reno, Nev., Jan. 10-13, 1983, the entire disclosure of which is incorporated by reference herein.
Regarding the reflection or deposition on a surface, when a moving particle impacts a surface, energy is lost through the compression of the particle and the surface. Some theoretical treatment of this phenomenon exists in the literature such as C. Thornton et al., “A theoretical model for the stick/bounce behavior of adhesive, elastic-plastic spheres”, Powder Technology 99 (1998), pp 154-162 the entire disclosure of which is incorporated herein by reference. Kinetic energy is also converted between translational and rotational modes as the particle tumbles. The tendency to tumble is a function of surface roughness, particle shape, and impact angle.
Whether a particle impacting a surface will stick is determined by whether the potential energy stored in compression is great enough to overcome the forces holding the particle to the surface. Particles on a surface are held to the surface by several forces, but the large forces are electrostatic attraction, surface energy, and surface tension.
If the particle bounces, the direction of the bounce may be affected by rotational momentum, surface roughness, and particle shape.
The prior art is lacking in a model for this last phase of particle transport. The present invention provides a comprehensive approach for describing and modeling the interactions between a particle and a surface.